The invention relates to an electric machine rotor according to the preamble of claim 1.
In an electric machine rotor provided with a plural number of magnetic poles, arranged substantially at equal intervals along the circumference of the rotor, each magnetic pole has a direct pole axis. Two adjacent direct pole axes form an angle that is bisected by a quadrature axis. The reactance corresponding to the direct pole axis is called a direct-axis reactance and the reactance corresponding to the quadrature axis a quadrature-axis reactance.
There are many types of electric machines in which an effort is made to minimize the quadrature axis flux, i.e. to maximize the ratio of the direct-axis reactance to the quadrature-axis reactance. Examples of these electric machines include the reluctance motor and the synchronous induction motor.
Publication GB 940,997 discloses a rotor of a synchronous induction motor provided with flux paths extending between adjacent magnetic poles. The flux paths are formed with flux barrier slots of low permeance extending between adjacent flux paths. Between adjacent flux paths there are a plural number of ‘necks’ for mechanically holding the rotor together. The necks between the flux paths are made of the same material as the flux paths themselves, i.e. of a material, such as iron, having high permeance. A problem with this type of rotor construction is that the necks of high permeance cause leakage flux between the flux paths, i.e. the necks impair the ratio of direct-axis reactance to quadrature-axis reactance.
The higher the speed of rotation of the electric machine, the wider the necks between the flux paths have to be in order to hold the rotor together. Making the necks wider increases leakage flux, because it further decreases magnetic resistance between the flux paths at the necks.